Numerous derivatives of organic phosphinic and dithiophosphinic acids are known to exist and to have considerable commercial value as well as a great variety of useful applications. For example, organic phosphinates and dithiophosphinates as well as their acids are effective wetting agents and detergents; plasticizers for many plastics and resins; bonding agents for asphalt and similar compositions; color stabilizers and oxidation inhibitors for greases and lubricants (U.S. Pat. No. 3,001,938); corrosion inhibitors; flame proofing agents; flotation auxiliaries; metal extractants; setting retarders for gypsum; and textile auxiliaries such as filament stabilizers (U.S. Pat. No. 3,374,288).
Highly purified, highly branched dialkyldithiophosphinic acids have been especially recognized as being very important and much desired precursors, intermediate products, and end products in numerous specialized fields, including mining. For example, branched dialkyldithiophosphinic acids act as metal extractants and froth flotation agents.
As a result of the above listed numerous possibilities of practical application, a demand has been created for a simple industrial synthesis for the production of dialkyldithiophosphinic acids in a highly purified state.
Because of the above described great commercial value, many methods of preparing organic phosphinic acids have been advanced. Although the methods vary widely in their individual steps, a great many employ the reactions of phosphorous-halogen compounds to attain carbon-to-phosphorous bonds. While it has long been known to be possible to form such bonds by reacting alkyl halides with phosphine, or by the use of Grignard reagents, such methods are not practical in commercial scale operations.
Stiles et al. (U.S. Pat. No. 2,724,718) discloses a process for the production of phosphinates employing the reaction between a compound containing olefinic double bonds and, preferably, a class of compounds consisting of compounds of the formula (I):
wherein Z represents a monovalent hydrocarbon radical free of aliphatic multiple bonds, or a monovalent inorganic cation, and Y represents a hydrogen atom, a monovalent hydrocarbon radical free of aliphatic multiple bonds, or the group —OZ in which Z is defined as above. Among the phosphorous classes and compounds that Stiles et al. suggest as reactants are the salts of hypophosphorous acid, hydrocarbyl esters of hypophosphorous acid, hydrocarbyl esters of organic phosphinic acids and mono- and di-hydrocarbyl esters of phosphorous acid. A particularly preferred subclass comprises the alkali metal salts of hypophosphorous acid such as sodium hypophosphite which Stiles et al. found to be able to be directly added to olefins containing up to 14 carbon atoms “to produce in a single, operational step a water soluble detergent in substantially quantitative yields.”
Stiles et al. also noted that 1-olefins exhibit a somewhat higher rate of reaction in these processes than do other olefins. The Stiles et al. addition reaction is initiated by the presence of free radicals in intimate contact with the reactants. Neither the reaction temperature nor the reaction pressure is taught to be critical by Stiles et al.
Stiles et al. teach that where a mole to mole addition is desired, it is generally preferable to employ the reactants in about equimolar proportions or with the phosphorous compound in excess; and, where it is desirable to cause more than one mole of the olefinic compound to be incorporated in the product.
Dialkyldithiophosphinic acids have also been used to extract metals (U.S. Pat. No. 5,447,552). In the general procedure employed for the separation of metal elements from solutions thereof, especially acidic solutions, the feed solution generally contains nickel and/or cobalt ions. The extract containing the extracted metal(s) is usually sent to a scrubber wherein it is scrubbed with dilute acid and then sent to a stopper where it is stripped with more concentrated acid to separate the metals. Hydrochloric acid and sulfuric acid are the preferred acids of the prior art to scrub and strip the extract. Bis-(2,4,4-trimethylpentyl) dithiophosphinic acid is said to be a preferred extractant; especially for the separation of cobalt from nickel.
The currently commercialized method to manufacture dialkyldithiophosphinic acids is by direct sulfurisation of the corresponding dialkyl phosphines, these are in turn manufactured from phosphine gas and olefins under pressure. Phosphine is highly toxic and flammable and the addition of an olefin to phosphine gas often yields a mixture of primary, secondary and tertiary phosphines. In particular when a dialkyl phosphine is the desired product the process also produces undesirable amounts of trialkyl phosphines which unlike primary phosphines, cannot be recycled and, on sulfurisation, produce trialkyl phosphine sulfides as an impurity, which must be removed by extraction from the dialkyldithiophosphinic acids.
This art-recognized problem of producing high purity dialkyldithiophosphinic acids by a practical reaction process which is applicable to the production of compounds having a variety of structures, especially highly branched dialkyl structures, has heretofore remained unsolved.
Phosphinic acid was converted by reacting with thiophosphoryl chloride into thiophosphinic chloride at very low yield (33-54%, Fedorova, G K et al Zhurnal Obshchei Khimii (1982), 52(1) 214 Chem. Abst. 96:181355.
Alternatively, thiophosphinic halides can be prepared from other starting materials such as phosphines (Kaushik, M. P. et al Indian J. Chem. Sect B: Org. Chem. Med. Chem. (1991), 20B(10), 932), thiophosphonous anhydride (Bliznyuk, N. K. et al SU347332), alkaryl halide (Baranov, Yu. I. et al SU221695), phosphinic chloride (Groenweghe, L. C. C. U.S. Pat. No. 3,206,442; Kabachnik, M. I. Godovikov, N. N. Doklady Akademii Nauk SSSR (1956), 110 217 Chem abst. 51:25335), dithiophosphinic acid (Lorenz, W. Schrader, G. DE 1067017) and bis(dialkylphosphinothioyls) (Colin, R. DE 1054453).
Accordingly, it is an object of this invention to provide a practical and efficient process for addressing this technical problem of producing high purity dialkyldithiophosphinic acids by providing conditions whereby, in a straightforward alpha olefin-hypophosphorous acid or a salt thereof free radical reaction, any monoalkylphosphinic acid and other water soluble impurities present are removed from the dialkylphosphinic acid product by a simple neutralization/phase separation without the need for a third component organic solvent addition before converting the dialkylphosphinic acid to dialkyldithiophosphinic acid.
Other objects will be evident from the ensuing description and appended claims.